The present invention relates to an electrophotographic image forming apparatus and a developing device used for the image forming apparatus
As a development method adopted for the electrophotographic image forming apparatus, a monocomponent development method using only a toner as a main component of developer, and a two-component development method using a toner and a carrier as main components of developer are known.
The developing device of the monocomponent development method has a toner bearing member which bears and conveys a toner and a friction charge member which comes into contact with the toner bearing face of the toner bearing member. Upon passing through a contact position with the friction charge member, the toner borne by the toner bearing member comes into frictional contact with the friction charge member so as to be formed into a thin layer and charged to predetermined polarity. Thus, in the monocomponent developing device, a toner is charged by frictional contact with the friction charge member, which brings such advantages that the structure can be simple, small, and inexpensive. However, since the toner is subjected to strong stress at the contact position with the friction charge member, the toner is prone to deterioration, and therefore the chargeability of the toner is damaged at a relatively early stage. Moreover, the toner is attached to the toner bearing member and the friction charge member due to the contact pressure therebetween, by which toner charging performance is degraded and as a result, the life of the developing device becomes relatively short.
The developing device of the two-component development method supplies a toner from a magnetic brush of the developer retained on a developer bearing body to an electrostatic latent image on the image bearing body for performing development. Since a toner and a carrier which constitute the developer are charged to predetermined polarity by frictional contact therebetween in the developing device, the stress exerted to the toner is smaller than that in the case of the monocomponent developing device. Since the surface area of the carrier is larger than that of the toner, the carrier is free from becoming dirty due to adhesion of the toner. However, the two-component development method had an inconvenience, that is, when a magnetic brush is directly brought into contact with an image bearing body for carrying out development, the magnetic brush causes irregular sweeping, resulting in sliding noise generated in images.
From the viewpoint of taking advantage of both the monocomponent development method and the two-component development method, a developing device of a so-called hybrid developing method is described in JP 56-40862 A and JP 2006-308687 A, in which charging of toner is performed in two-component method involving small stress, while development of electrostatic latent images is performed in monocomponent development method in which fogging is relatively small. In this hybrid developing method, the toner with relatively large particle size tends to be selectively presented from the toner bearing body to the electrostatic latent image, and therefore when continuous printing is performed, the toner having relatively small particle size and charged to high potential tends to accumulate on the toner bearing body and to cause selective development, which fosters a tendency of lowering the density in images to be formed. Therefore, if there are a section (undeveloped section) where the toner was not presented for development and a section (developed section) where the toner was presented and consumed for development on the toner bearing body, only a low-charged toner, which is easily scraped in a mechanical manner by the magnetic brush on the developer bearing body, is collected among the toners in the undeveloped section, and a high-charged toner is left uncollected, while the toner with an average charge amount is newly supplied to the toner bearing body in the developed section from the magnetic brush. This causes such a problem as easy generation of a so-called hysteresis phenomenon in which some of the last developed image appear as an afterimage (memory image) at the time of next development. In a concrete example, when a rectangular gray halftone image 5 with a size large enough to contain a small rectangular black solid image 3 is formed next to the black solid image 3 as shown in FIG. 17A, a toner consumption area and a toner non-consumption area are generated on the toner bearing body, so that as shown in FIG. 17B, an afterimage 7 corresponding to the toner consumption area of the black solid image 3 appears in the halftone image 5.
An image forming apparatus disclosed in JP 2006-308687 A has a developing device composed of a magnetic roller and a developing roller. From developer containing a toner and a carrier retained on the peripheral face of the magnetic roller, only the toner is selectively supplied to the peripheral face of the developing roller, and an electrostatic latent image (electrostatic latent image section) on the photoconductor is developed using the toner retained on the peripheral face of the developing roller. In the invention of JP 2006-308687 A, the developer contains charged particles, which are present between the toner and the carrier without being retained on the surface of the toner nor the carrier, and which prevent pulverized toner powder from adhering to the surface of the carrier to form spent. However, the charged particles are contained only in the developer initially introduced to the developing device. Since the charged particles are not retained on the surface of the toner nor the carrier, some of them are supplied to the developing roller together with the toner because of their electric coupling with the toner, and then adhere to the nonimage section in an electrostatic latent image on the photoconductor where they are gradually consumed. Consequently, if a large quantity of an image with small image area ratio or small image ratio (so-called monochrome ratio), such as character images, are printed, then only the charged particles are consumed in large quantities, which causes a problem in obtaining the chargeability of the toner stable for a long time.
In order to solve the problem of the hysteresis phenomenon in the hybrid developing method mentioned above, it is necessary to improve toner recoverability from the toner bearing body or from the developing roller at the position after the developing area. To this end, it is possible to consider adjusting such conditions as placement of magnetic poles of the magnetic roller as a developer bearing body, developer transportation quantity, and distance to the developing roller, so as to increase the developer density between the developing roller and the magnetic roller in order to enhance the efficiency of toner recovery from the developing roller. However, when the developer density between both the rollers is increased, problems such as torque increase and heat generation by clogging of the developer arise.
In order to enhance the toner recoverability from the developing roller in the hybrid developing method, it has been proposed in JP 2003-280357 A to form an oscillating electric field between the developing roller and the magnetic roller, which acts in favor of toner recovery. However, this proposal impairs the original function, that is, to supply a toner from the magnetic roller to the developing roller. Accordingly, it has been proposed in JP 2005-10290 A to activate an electric field which electrically collects the toner on the developing roller at the time of non-image formation after the end of image forming operation. However, this causes a problem in which the carrier on the magnetic roller tends to be electrostatically adsorbed to the developing roller in connection with the complication of bias control and application of recovery bias.